The present disclosure relates generally to methods for forming activated carbon, and more particularly to microwave energy-assisted, chemical activation of carbon using plural, discrete heating steps.
Energy storage devices such as ultracapacitors may be used in a variety of applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Ultracapacitors typically comprise a porous separator and an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the electrochemical double layers that are created at the interfaces between the electrodes and the electrolyte. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon that is incorporated into the electrodes.
Carbon-based electrodes suitable for incorporation into energy storage devices are known. Activated carbon, which forms the basis of the electrodes, can be made from natural or synthetic precursor materials. Natural precursor materials include coals, nut shells, and biomass. Synthetic precursor materials typically include phenolic resins. With both natural and synthetic precursors, the activated carbon can be formed by carbonizing the precursor and then activating the intermediate product. The activation can comprise physical (e.g., steam or CO2) or chemical activation at elevated temperatures to increase the porosity and hence the surface area of the carbon.
Both physical and chemical activation processes typically involve large thermal budgets to heat and react the carbonized material with the activating agent. In the case of chemical activation, corrosive by-products can be formed when a carbonized material is heated and reacted with a chemical activating agent such as KOH. This can add complexity and cost to the overall process, particularly for reactions that are carried out at elevated temperatures for extended periods of time.
Accordingly, it would be an advantage to provide activated carbon materials and processes for forming activated carbon materials using a more economical chemical activation route. The resulting activated carbon materials can possess a high surface area to volume ratio and can be used to form carbon-based electrodes that enable efficient, long-life and high energy density devices.